Some people walk out of screenings of Etre et Avoir. It is, after all, very, very slow. It has a running time of 104 minutes, but possesses a Tarkovskyan stateliness that makes it feel much longer. The first scene is of cattle moving unhurriedly across an Auvergne snowscape. The second is of tortoises crawling across an empty classroom floor. Perhaps not since the time Terence Davies let his camera wander across the pattern of a carpet in The Long Day Closes have there been cinematic scenes that would so annoy the impatient viewer.

What's more, this is a documentary about a very specialist subject: single-classroom schools in which one schoolteacher instructs all the children from the same village, from nursery age to their last year of primary education. Even among French educationalists, these schools are going out of fashion. On the face of it, this is a minority-interest film and then some. That it has been given a cinematic release in Britain at all is surprising. And yet, Etre et Avoir is charming - and arrives in Britain trailing a string of awards.

The documentary's hero is 55-year-old Georges Lopez, a gentle man in his penultimate year of teaching. He is the pedagogical flip side to Brian Glover's bullying, self-aggrandising sports teacher in Ken Loach's Kes; the kind of teacher anyone would wish that they or their children could have had. Through the character of Lopez and his work with the children, Etre et Avoir taps deep emotions about, and memories of, childhood.

It is directed by Nicolas Philibert, who has been planning this project for some time. "Years ago I wanted to make a film about learning to read," he says, "which might have resulted in a documentary that is like watching paint dry." He realised that a film about schools might still prove problematic for some viewers. "I needed to slow the audience down, to adjust them to the temporal world of my film. That's why I started with the cattle and the tortoises. Also, those scenes have a symbolic resonance: the children are gently herded like cattle, and as they learn, they make slow but steady progress in reading and counting like tortoises. That, at least, is what I hoped they would suggest."

Philibert spent a great deal of time finding the right school for the film, one with a manageable class size of 10 to 12 pupils so that each child could become a character, and one with the widest possible age range in order to show the demands put on teachers. "I considered 300 schools and none quite measured up. One would have too many children, another would be opposite a noisy building site that would have ruined the film. I was beginning to despair when, one cold November day in 2000, I arrived at the school in the village of Puy-du-Dome and, after a quarter of an hour, I was convinced that I had found my class.

"I was immediately seduced by the personality of the schoolmaster in whom I detected, beneath his slightly authoritarian air, a profound attentiveness. I had a little digital video camera that I brought out of my pocket each time I thought I was on to something. While making images of the class, I understood that the teacher was not trying to show himself in a good light: no propaganda, no hot air. He established himself at once as a strong character. And then there were the children, their faces tense with the desire to go forward to the future."

Philibert works in a way reminiscent of British documentary-maker Michael Waldman, who is most notable for The House, the BBC2 series about the machinations at the Royal Opera House in London. Waldman attempted to gain the confidence of everyone, and then blend with the woodwork in order to capture the most poignant and revealing moments. Philibert has done much the same in the cinema, in such unpromising-sounding films as La Ville Louvre, about the museum after hours; The Land of the Deaf, about people who have been deaf since birth; and La Moindre des Choses, about a group of mentally ill patients rehearsing and performing an operetta.

For Etre et Avoir, Philibert and his four-strong crew spent seven months in the classroom amassing 600 hours of footage that he edited into a coherent film spanning the school year and the seasons. Did he play with the children? "Only during school break. When we were in the school room, we arranged it so that they ignored us and treated us like a piece of the furniture."

What we see are compelling quotidian dramas and little vignettes of learning: a girl struggling to trace letter shapes, a baffled boy who counts as far as he can and then wonders if there are any bigger numbers. So far, so cute. But the best scenes involve the endlessly dedicated Lopez, who is especially winning when he counsels two boys, Julien and Olivier, to understand why they got into a schoolyard brawl. "You were just testing each other, but then it degenerated?" he asks. The boys nod sulkily. In another scene, Lopez sits under a tree with Julien and talks to him about the boy's father, who has just undergone a life-threatening operation: something that it is hard to believe many teachers would have the time or the temperament to do.

That said, the unfolding drama can at times feel a little too wholesomely twee: little four-year-old JoJo, with his dirty hands, is almost too sweet for the audience's good. You can't help but wonder why Philibert set his film in an idyllic, overwhelmingly white, rural school rather than getting to grips with educational problems in, say, the tough Parisian banlieues, or with the difficult relationships explored in the Dardenne brothers' Le Fils (The Son), set in a Belgian vocational school.

Etre et Avoir bears most fruitful comparison with Bertrand Tavernier's 1999 film It All Starts Today (Ça Commence Aujourd'hui), about a kindergarten teacher in a town whose economy and spirit is ruined by the closure of its coal mine. But Tavernier's was a film that dealt with French public services in extremely difficult circumstances; Philibert's documentary inhabits a different world altogether. Originally, he says, "I wanted to set the film in a mountainous region where the climate is harsh and the winter is difficult. That's because I had this idea about making a film about the farmers on the edge of bankruptcy. Then by chance I got interested in these schools." But ultimately his documentary is located in a place that finds public services working as blissfully as they ought, in a society whose problems are relatively minor.

Doesn't Etre et Avoir just produce in its audience a soupy nostalgia for la France profonde, rather akin to the cinematic Provence in Claude Berri's Jean de Florette and its sequel? Philibert bristles: "It's nothing like that. I don't think it evokes nostalgia. Anyway, perhaps this film can serve as a kind of model for what schooling can and should be like."

In any case, such objections get washed away by the film's denouement. On the last day of the school year, when the children line up to kiss Lopez farewell, there are tears everywhere: from Lopez, from the little boys and girls who are saying goodbye for the last time, for those of us in the audience who didn't walk out during the first reel. And for Philibert, too, perhaps? "Oh no, we were too busy filming it. We're professionals, you know."